Determination of levamisole, aminorex, and pemoline in plasma by means of liquid chromatography-mass spectrometry and application to a pharmacokinetic study of levamisole.

Levamisole is an anti-helminthic drug and gained forensic interest after it was found that it was used as a cocaine adulterant. A liquid chromatography-mass spectrometry (LC-MS) method for the determination of levamisole and its metabolite aminorex in human plasma is described. Selectivity is given; calibration curves were linear within a calibration range of 1 ng/mL-500 ng/mL. Limits of detection and quantification (LODs, LOQs) were 0.85 ng/mL for levamisole and 0.09 ng/mL, and 0.34 ng/mL for aminorex, respectively. Precision data was in accordance with the GTFCh guidelines. The validated method was successfully applied to study the pharmacokinetics of levamisole after administration of 100 mg of levamisole orally. Levamisole could be detected up to 36 h after ingestion in serum, while aminorex never exceeded the LOQ. A one-compartment model best described levamisole pharmacokinetics. The following parameters were calculated: ka = 1.2 [1/h], CL/F = 52 l/h, V/F = 347 l, f (renal) = 0.0005, t ½ = 2.0 h, AUC = 1923 ng/mL*h, cmax = 214 ng/mL, tmax = 1.98 h. Levamisole could be quantified in 42.5% of cocaine--positive plasma samples (2.2 to 224 ng/mL). Aminorex was positive in only 11.3% of the cases; however, it was never found higher than the LOQ. Pemoline, another stimulant detected in horse urine samples after administration of levamisole, was not found either in serum or in urine of this pharmacokinetic study. In post-mortem cases, levamisole and aminorex could be detected in femoral blood and the urine of cocaine users. Pemoline was not detected.

Metabolism of levamisole and kinetics of levamisole and aminorex in urine by means of LC-QTOF-HRMS and LC-QqQ-MS.

The antihelminthic drug Levamisole can enhance cocaine effects by conversion into the amphetamine-like drug aminorex. We describe an LC-MS method for the determination of levamisole and its metabolite aminorex in human urine. Selectivity is given, calibration curves were linear within the calibration range 2.5-250 ng/mL; limits of the method were LoD 0.51 ng/mL, LoQ 1.02 ng/mL for levamisole and LoD 0.65 ng/mL, LoQ 0.76 ng/mL for aminorex. Precision data was in accordance with the guidelines (intraday precision for aminorex ranged between 5.75 and 11.0 % for levamisole between 8.36 and 10.9 %; interday precision for levamisole 10.9-16.9 % and for aminorex 7.64-12.7 %; accuracy data for levamisole -1.96 to -14.3 % and for aminorex-11.9 to-18.5 %). The validated method was successfully applied to study the urinary excretion of levamisole after the administration of 100 mg of levamisole orally. Levamisole and aminorex could be detected in post-administration urine samples. Levamisole could be detected up to 39 h after ingestion, while aminorex was detectable up to 54 h. Maximum aminorex concentrations were 45 ng/mL urine. Further metabolites of levamisole after oral ingestion by means of liquid chromatography hybrid quadrupole time-of-flight high-resolution mass spectrometry (LC-QTOF-HRMS) were identified. Only 0.5 % of the ingested drug was quantified as unchanged levamisole in urine. Besides aminorex, five isomers of aminorex and 4 hydroxy-metabolites of aminorex or its isomers were found. Furthermore, levamisole is also hydroxylated and eliminated free or conjugated with sulfate or glucuronide into urine.